In the control of motors and power supplies it is desirable to use driving circuitry having a pulse width modulated (PWM) signal output. Conventional PWM controllers, for example fixed frequency PWM controllers, regulate power converters by controlling the duty ratio or duty cycle of the power converter via the output voltage. Fixed-frequency PWM controllers can produce significant phase lag between the PWM output signal and the output voltage. Such phase lag typically is caused by the L-C portion of the control-to-output transfer function. Such controllers require careful design to ensure stability and adequate response speed for the various operating conditions which they may encounter.
It has been recognized by Redl, R. et al. in "What A Design Engineer Should Know About Current-Mode Control", printed as part of the Proceedings of Power Electronics Design Conference, Oct. 15-17, 1985, pages 18-33, published by the Power Sources Users Conference of Cerritos, CA, USA, that phase lag problems can be overcome through the use of current-mode control rather than voltage control. Current-mode controllers overcome the phase lag problems of voltage control converters by directly commanding the current in the power converter inductor. Such direct command allows the inductor current to instantaneously follow the control input, thus removing the phase lag problem. The prior art recognizes at least five different types of current-mode controllers (a) hysteretic, (b) constant OFF time, (c) constant frequency with turn-on at clock time, (d) constant ON time and (e) constant frequency with turn-off at clock time. Among these, the hysteretic constant OFF time, and constant ON time are variable-frequency. These five types of controllers are used in combination with three basic types of regulated rectangular wave power converters, namely buck, boost and buck-boost. Of the above five current-mode controllers, it has been found that the hysteretic controller is the best in three respects: largest phase margin, freedom from subharmonic oscillations, and well limited short circuit current.
Some variable frequency techniques for PWM establish control of the peak inductor current and always allow the inductor current to return to zero, i.e. they are always in a discontinuous-current mode. Neither the ON (conductive) time nor the OFF (nonconductive) time are fixed. Rather, the ON time is terminated when the sensed current reaches a fixed value. The error signal controls the frequency of operation directly through use of a voltage-controlled oscillator. This type of PWM control exhibits desirable characteristics at light loads. However, this mode of operation is inefficient under heavy load conditions because of the high peak-to-average current ratio. It is also undesirable in motor applications, since the relatively high peak current may tend to demagnetize the rotor. Under heavy load conditions, operation in a hysteretic mode is desirable. In the hysteretic mode, the peak-to-average current ratio is controlled to a lower value than in the variable-frequency controller.
In a typical hysteretic current-mode controller, a voltage representative of the inductor current in the power converter is compared in a comparator with the error signal. The power converter may be the drive circuits for a motor or a switched direct current (dc) power supply. In the power converter, the error signal may be a voltage representing the difference between the commanded value of a parameter and its actual value. Controlled parameters in power supplies may be direct output voltage, or in a motor the controlled parameters may be torque or speed. The comparator renders a power switch in the power converter nonconductive (OFF) when the increasing sensed inductor current reaches a design value which is a predetermined amount above the error signal. The comparator turns the power switch ON again (renders it conductive) when the sensed inductor current decreases a predetermined amount below the error signal. The current difference between the turn-OFF and turn-ON points defines a hysteresis band, which may be symmetrical about a reference value.
Hysteretic current-mode controllers are preferred for heavy inductor current demand conditions, i.e. continuous inductor current conditions, as required, for example, for a motor being started. Hysteretic current-mode controllers may be unsuitable for light inductor current demand conditions, i.e. discontinuous inductor current conditions, as required, for example, by a motor in operation at relatively constant speed. A discontinuous inductor current condition is one in which the inductor current drops to zero during a portion of the operating interval. During such discontinuous modes of operation, hysteretic controllers may be subject to undesirable low frequency oscillations.
A previously suggested solution to the problem of oscillations at low load in hysteretic controllers is the combination of a hysteretic controller with a constant OFF time controller. However, such a combination is undesirable, as described below. The operation of a typical constant OFF time current-mode controller is similar to that of the hysteretic controller, except the power switch is turned OFF for a fixed time period. The ON-time of the power switch is terminated when the voltage representative of the inductor current reaches the value of the error signal. The fixed OFF time of such a controller is accomplished through the use of a monostable multivibrator set to a predetermined time period. Thus the constant OFF time current-mode controller controls the maximum value of the inductor current, while the hysteretic current-mode controller controls both the maximum and minimum values of the inductor current. Both result in variable-frequency operation.
The problem with combining a hysteretic controller with a constant OFF time controller is that the ON time can be so low during light inductor current conditions, i.e. at minimum torque in a motor, that the ON time approaches the storage time of the power transistor of the converter, which may result in an oscillation condition. As mentioned, such oscillations are disadvantageous. Thus, the constant OFF time controller used to alleviate the problems of the hysteretic controller under low load conditions itself has problems at low load. It is therefore desirable to have a current-mode controller which provides stable and efficient operation at all loads.